Field of the Invention
The invention relates to a pipe having ribs on its inner surface forming a multiple thread, and steam generators and heat transfer systems using the pipe.
Pipes that are ribbed on the inner surface have been used for some time in steam generators, in order to control major thermal strains in the combustion chamber. For instance, an article entitled "Zwangdurchlaufkessel fur Gleitdruckbetrieb mit Vertikaler Brennkammerberohrung" [Forced Circulation Boiler for Sliding Pressure Operation with Vertical Combustion Chamber Piping] by H. Juzi, A. Salem and W. Stocker, published in VGB Kraftwerkstechnik [VGB Power Plant Industry] 64, pages 292-302, describes on page 294 thereof, among other things, that on one hand in a region of high combustion chamber thermal strain with smooth evaporator pipes, film evaporation must be expected within a wide range of subcritical pressures, yet on the other hand with pipes which are ribbed on the inner surface, the film evaporation is limited to a pressure range between approximately 206 bar and the critical pressure.
Due to the film of vapor between the metal pipe wall and the liquid phase of the heat absorption medium, the film evaporation hinders the heat transfer, so that the pipe wall temperature rises sharply in the region of the film evaporation. In steam generators with forced circulation of a coolant, the film evaporation occurs practically only in the region in which both the liquid and the vapor phase of the coolant occur simultaneously. Tests have confirmed that with smooth-walled pipes, film evaporation must be expected even with low steam content, and that the evaporation shifts to higher steam contents when pipes that are ribbed on the inner surface are used. This shift simultaneously reduces the extent of the undesired temperature increase in the metal pipe wall.
As can be found both in the above-cited article and in a report presented to The International Heat Transfer Conference in Tokyo, September 1974, paper PGTP 73-54, pages 14-21, the desired shift in the film boiling in commercially available internally grooved pipes, occurs only at relatively high mass flow densities and high coolant speeds.